US race to trace the source

The US Government is depending on a mix of quarantine and detective work to figure out how mad cow disease apparently infected a cow in Washington state.

The nation's mad cow emergency plan - never before used - is to cordon off any cattle that could have come into contact with the infection.

Keeping any possibly infected cow out of the food chain is crucial, as a human version of the disease that literally eats holes in the brain is believed spread by eating meat tainted with infected brain or nerve tissue.

The human illness, Creutzfeldt-Jakob disease, has claimed 143 victims in Britain and 10 elsewhere, but none so far in the US.

Already, the farm near Yakima, Washington, has been quarantined as the Government awaits the last of three tests on the suspect cow's tissue to confirm the presence of bovine spongiform encephalopathy (BSE). The final test is performed in Britain, where BSE first affected cattle in 1986.

Once the disease was confirmed, all animals in the herd probably would be slaughtered so their brains could also be tested, said Food and Drug Administration deputy commissioner Lester Crawford, a veterinarian who oversees the agency's BSE work.

The key is to trace the source of the first infection to see how the US preventative measures against the disease was breached.

Beef and cattle imported from countries known to have BSE have long been banned.

But the nation's main defence is a 1997 ban on giving cattle feed made from the protein or bone meal of sheep or other mammals - because this feed is believed to be the way mad cow disease originally spread.

So the first question is whether the cow was illegally imported or ate feed that illegally contained BSE-bearing protein.

Another possibility depends on the cow's age, which was not immediately known. If the cow was more than six years old, she could have received tainted feed before the FDA's ban began, Mr Crawford noted.